Sub testdpt()
Dim w As Worksheet
Dim r As Range
Dim c As Range
Dim s As String
Set r = Worksheets("EtatPresence").Range("C5")
Set r = r.Resize(r.End(xlDown).Row - r.Row + 1)
For Each c In r.Cells
Set w = Nothing
On Error Resume Next
Set w = Worksheets(c.Value)
On Error GoTo 0
If w Is Nothing Then s = s & c.Value & vbCr
Next c
If s <> "" Then MsgBox "Manque : " & vbCr & s
End Sub